Microchip Technology

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  • Zener Diode Tester

  • Created: May 23, 2016

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Device testing can be functional and parametric. A functional test determines whether or not the device works well enough for the intended use while parametric test measures all device parameters to see if they meet the specified values. This reference design is a simple Zener diode tester that is both functional and parametric. It reads the voltage drop across the device under test where unknown Zener diode’s characteristics can also be identified. This device specifically uses MCP87090 N-channel power MOSFET of Microchip that has low drain-to-source on resistance and low total gate charge.

This reference design a simple dc-dc boost converter designed with 555-timer. The 555-timer is wired as an astable oscillator at a frequency around 68KHz. Its output directly drives the power MOSFET (U1) that switches the current through power inductor (L1). The diode D1 and capacitor C3 form the output rectifier and filter circuit. The Zener diode D2 sets the output voltage to near 33V, and resistor R3 is the ballast resistor for limiting test current through the device under test. The DVM reads the voltage drop across the device under test. If the readout is >30V probably, the Zener Diode Under Test (ZUT) is open or has a higher value. Meanwhile, if it is < 1V, then the ZUT is short.

This circuit is suitable for testing Zener diodes of up to 30V rating. It can also be used to check the forward voltage drop across diodes and LEDs. The test current through the device under test can be set to a different value by changing the value of the ballast resistor (R3).